This invention relates to an apparatus for the extrusion of synthetic filaments. In particular, it relates to a spin pack assembly for the melt spinning of artificial filaments from thermoplastic materials such as nylon and polyester.
Although it will be understood from the following description that the present invention may be successfully used to produce fibers from various materials, it it nevertheless particularly applicable to the production of synthetic polymer fibers, and accordingly, stress will be placed on the use of polymer in the following description.
Conventional spin pack assemblies comprise a top cap, a filter assembly formed by a series of fine metal gauzes or fine refractory material which are held in position by metal retaining gauzes, a perforated breaker plate arranged so as to maximize mixing and minimize stagnation of the polymer and which supports the metal retaining gauzes of the filter assembly, an optional filter positioned next downstream from the breaker plate, a spinneret plate provided with shape imparting polymer flow channels and to which the melt is passed from the optional filter or directly from the breaker plate, and a spin pack body which supports the spinneret plate.
During the extrusion process within the spin pack assembly, it is important that zones of zero or low velocity polymer flow be eliminated so as to facilitate the uniform flow under pressure of the polymer. These zones occur wherever a flat surface perpendicular to the polymer flow inhibits the smooth, downward flow of the polymer, particularly on the top surfaces of the breaker plate and spinneret plate and in the breaker plate-spinneret plate area. There are only a finite number of shape imparting polymer flow channels provided in the breaker plate and spinneret plate, and the surface area between these polymer flow channels provides a plateau perpendicular to the polymer flow upon which polymer can accumulate. The polymer builds up on these plateaus until it forms small, semi-stagnant pools which become thermally degraded. These pools, or portions thereof, intermittently dislodge themselves from the surface area and enter the polymer stream, thus, either causing a break in extrusion or contributing to the formation of weak points in the filaments.
Also, the pools, or portions thereof, which do not dislodge themselves from the surface area necessitate stopping the extrusion process periodically so that they may be removed, and then restarting. Whenever a break occurs in the extrusion process, it is usually necessary to spin filaments to waste for about 15 minutes before operation returns to normal. This is a serious disadvantage economically.
In the space between the breaker plate and spinneret plate there are pressure differences which affect the quality of the extruded polymer filaments. When the polymer flow channels of the breaker plate are capillaries whose exit is such as to form a right angle with the bottom surface of the plate, the polymer stream will deviate from its normal downward flow by up to this 90.degree. angle, thus creating pressure differences in the breaker plate-spinneret plate area. The greater the angle of deviation, the greater the pressure drop. Free polymer flow in this area would provide a more nearly equal pressure on the polymer entering the spinneret plate.
The prior art discloses conically shaped inlets to the polymer flow channels of the breaker plate and spinneret plate, as well as conically shaped outlets from the polymer flow channels of the breaker plate. These conically shaped inlets and outlets reduce the surface area upon which polymer can accumulate, and additionally, furnish a guiding means for the polymer flow. In a plan view of the breaker plate or spinneret plate these conically shaped inlets may be represented by an arrangement of circles within a bounded area. It is obvious that no matter how closely these circles are placed, relatively extensive perpendicular surface area remains upon which polymer can accumulate and inhibit a continuous polymer sweep.
Although the conically shaped outlets of the breaker plate disclosed in the prior art reduce pressure differences in the breaker plate-spinneret plate area by providing a smaller angle of deviation from the normal downward polymer flow, they still form an angle with the bottom surface of the breaker plate about which the polymer must flow; thus, pressure differences still exist. To eradicate them, a greater area of free polymer flow must be provided.